Analog-to-digital (A/D) converters form a digital signal from an analog signal. The basic steps required to convert an analog signal to a digital signal include: 1) bandpass filtering the analog signal, 2) sampling values of the analog signal, 3) quantizing the sample values, and 4) encoding the sample values into digital words composed of binary digits (bits). The basic components used to execute these three steps respectively include: a sampler, a quantizer and an encoder.
The sampling step is typically accomplished by a device known as a sampler. The sampler extracts sample values (usually measured in volts) from the input analog signal at predetermined sampling times. Each sample value is held at a constant value until the next sampling instant. This constant value is input to the quantizer.
Quantization entails assigning each sample value to one of the finite number of output values (known as "quantization levels") available to the quantizer. For example, suppose a quantizer includes the following four quantization levels: 0.25 volts, 0.5 volts, 0.75 volts and 1.0 volt. The output of this quantizer is limited to one of these four values and each sample value input to this quantizer is assigned to one of these four quantization levels. If an input value is not equal to one of these four available quantization levels, the quantizer may "round" the input value to the closest of these four levels. Thus, an input of 0.5 volts may be assigned to a quantization level of 0.5 volts; an input value of 0.45 volts may also be assigned to a quantization level of 0.5 volts; and an input value of 0.1 volts may be assigned to a quantization level of 0.25 volts. As this example illustrates, rounding is a source of error in a quantizer's output. This error can be reduced by increasing the number of quantization levels within a given range of input values.
The encoder assigns each quantization level received from the quantizer to a corresponding digital word. Thus, an encoder connected to the quantizer described above would include one unique digital word which corresponds to each of the four quantization levels. The digital word assigned to 0.25 volts might be "00"; the digital word assigned to 0.50 volts might be "10"; the digital word assigned to 0.75 volts might be "01"; and the digital word assigned to 1.0 volt might be "11". The relationship between the number of quantization levels and the length of the digital word required to uniquely represent each quantization level is given by the following expression: EQU q=2.sup.n
where "q" is the number of quantization levels and "n" is the number of bits in each digital word. Thus, to represent each of four quantization levels with a unique digital word, two bit (2.sup.2) digital words are required.
The "pipelined" or "successive approximation" converter is a known type of A/D converter. Successive approximation A/D converters utilize a single-bit quantizer to achieve multi-bit precision. In a successive approximation A/D converter, a comparator compares the input analog value to a predetermined voltage level (such as zero volts) and generates a digital "1" if the input value is greater than the predetermined voltage or generates a digital "0" if the input value is less than the predetermined voltage. This digital "1" or "0" is the first bit in the digital word which represents the input analog sample value. After this first bit is stored, this digital value is converted back to an analog signal using a digital-to-analog (D/A) converter so the signal may be used in the process of determining the second bit in the digital word which represents the same sample value. For example, a digital "1" is converted to a 1 volt analog signal and a digital "0" is converted to a negative 1 volt analog signal. A new input voltage to the A/D converter is then obtained by doubling the previous input voltage and subtracting off the D/A signal. The above process is repeated until the desired number of bits are obtained (for a given input sample).
The major advantages of the successive approximation architecture are a small chip area and low power consumption. However, since successive approximation A/D converters require a D/A converter in a feedback loop, it is necessary to wait for the output of the D/A converter to "settle" before the next bit can be obtained. The time required for settling restricts the speed of the A/D converter. All known A/D converters which use single-bit quantization to achieve multi-bit accuracy rely on feedback from the previous quantizer output.
In view of the above, it can be appreciated that there is a need for a method and apparatus which solves the above mentioned problems.